The Claus process is widely used by the industry for the production of elemental sulfur. The process is designed to carry out the Claus reaction: ##EQU1## The reaction is favored by decreased temperature and by removal of elemental sulfur vapor.
In the conventional (high temperature) Claus process, the operating conditions of the reactors in which the Claus reaction is carried out are selected to maintain elemental sulfur in the vapor state. Otherwise, the elemental sulfur would deposit on the catalyst and deactivate it. To assure high conversion rates, the reaction is carried out in a plurality of consecutive reactors. Elemental sulfur is condensed and removed from the effluent of a preceding reactor before the effluent is passed to a subsequent reactor. The removal of sulfur permits the reactors to be maintained at progressively reduced temperatures.
Generally, the desired recovery levels necessitate the use of a modified (adsorptive or low-temperature) Claus process which includes a thermal reactor, two catalytic reactors and two low temperature catalytic reactors, such as cold bed adsorption (CBA) reactors. The reaction in a CBA reactor is generally carried out at a temperature range which results in the condensation of elemental sulfur on the alumina catalyst, as an example, from about 250.degree.-280.degree. F. (121.degree.-138.degree. C). The low temperatures in the CBA reactor favor the reaction and the condensation of sulfur removes it from the reaction phase thereby allowing more H.sub.2 S and SO.sub.2 to react. The sulfur condensing on the catalyst deactivates it. Accordingly, a second CBA reactor is provided so that while the first CBA reactor is in the recovery mode, the second reactor is being regenerated and vice versa.
Although acceptable recovery rates can be achieved by the above-described modified Claus process, the capital expenditures and operation costs for such processes are quite high. One of the major factors contributing to the expense of such a process are the reactors. The elimination of one of the reactors from the modified Claus process would significantly reduce both overall captial expenditures and operation costs. However, this would in turn significantly lower the recovery rate thereby necessitating additional treatment of the tail gas to minimize pollution problems. Such treatment of tail gas by the conventional Scot or the Beavon process is expensive and consumes significant amounts of energy. Additionally, the lower recovery rate would decrease the overall recovery of elemental sulfur and hence adversely affect the economics of the process.
A number of processes have been designed in an attempt to achieve acceptable recovery rates at lower costs. Delta Engineering Corporation's MCRC Process uses three catalytic reactors. The first catalytic reactor operates above the dew point of sulfur throughout the process. One of the remaining two reactors, a low temperature reactor, is operated below the dew point of sulfur while the other reactor is being regenerated. When the catalyst in the low temperature reactor becomes deactivated due to sulfur deposits, the inputs to the two reactors are switched so that the catalyst in the reactor which was operated at a low temperature is regenerated by the high temperature gas and the reactor with the regenerated catalyst therein is operated at a temperature below the dew point of sulfur. The reactor operating in the regeneration mode is fed with a gas stream from the heat exchanger after the first catalytic reactor. The three catalytic reactors do not acheive acceptable recovery rates. Accordingly, in order to obtain the desired 98% average recovery rate, a coalescer must be added behind the third catalytic reactor. The coalescer, of course, adds to the overall cost of the system and therefore diminishes the savings achieved by the elimination of a catalytic reactor.
Another prior art process which attempts to acheive acceptable recovery levels at reduced cost is the Maxisulf system of Davy McKee AG. The Maxisulf process provides two low temperature reactors, one of which operates as the low temperature reactor while the other is regenerated. The regeneration of the low temperature catalytic reactor is accomplished by forcing therethrough a stream of hot gas from an inline burner. The hot gas flows through the reactor in the opposite direction from that of the low temperature gas.
This process suffers from several drawbacks. First, the alternating directions of flow accelerates the degradation of the catalyst. Second, the process utilizes an extra burner and fuel gas, both of which increase the overall cost. Third, the process introduces a dangerous condition. If oxygen is present in the regeneration gas stream, it can sulfate and therefore deactivate the catalyst. This problem can be overcome by contacting the regeneration gas with H.sub.2 S, but each step would further add to the cost of the process. Fourth, the quality of the fuel gas must be carefully controlled to avoid the introduction of unburned hydrocarbons into the low temperature reactor. Otherwise, the hydrocarbon would be cracked and produce a tar, which, in turn, would coat and therefore deactivate the catalyst. Finally, the inline burner adds an additional volume of gas which must be treated and processed.
Thus, the prior art processes have not been entirely successful in solving the problem of reducing the overall cost of the modified Claus process. There is therefore a long-felt and still unsatisfied need for a process and a system that would require fewer reactors than the conventional modified Claus process, but acheive the required overall recovery of sulfur without the need for a further treatment of the tail gas and without the need for additional expensive components of the system. The present invention achieves the above-stated goal.